In the drilling of oil and gas wells, downhole drilling motors may be connected to a drill string to rotate and steer a drill bit. Conventional drilling motors typically include a power assembly, a transmission assembly, and a bearing assembly. Rotation is provided by the power assembly. The transmission assembly transmits torque and speed from the power assembly to a drill bit disposed at a lower end of the drilling motor. The bearing assembly takes up the axial and radial loads imparted on the drill string during drilling.
A conventional bearing assembly, such as bearing assembly 10 shown in FIG. 1, includes mandrel 12 positioned through upper radial bearing 14, thrust bearing 16, and lower radial bearing 18. Lower end 20 of mandrel 12 is configured to engage a drill bit. Upper bearing housing 22 encloses upper radial bearing 14 and thrust bearing 16. Lower bearing housing 24 encloses lower radial bearing 18. Upper radial bearing 14 includes outer sliding member 26 and inner sliding member 28. Lower radial bearing 18 includes outer sliding member 30 and inner sliding member 32. Outer sliding members 26 and 30 each includes an inner surface having a flat profile, and inner sliding members 28 and 32 each includes an outer surface having a flat profile. The opposing flat profiles slide along one another as outer and inner sliding members 26, 28 and 30, 32 rotate relative to one another. Thrust bearing 16 includes a series of ball bearings 34 disposed within grooves formed by multiple outer thrust members 36 and multiple inner thrust members 38. Sliding radial bearings wear due to frictional forces that causes abrasive wear at the contact surfaces. The diameters of ball bearings 34 decrease as they are worn, which causes relative axial movement between outer thrust members 36 and inner thrust members 38. This relative axial movement in turn causes relative axial movement between outer sliding member 26 and inner sliding member 28 and relative axial movement between outer sliding member 30 and inner sliding member 32. Because the flat profiles of each sliding member allows the relative axial movement within upper radial bearing 14 and lower radial bearing 18, the radial bearings do not absorb any of the thrust load.
In other conventional bearing assemblies, radial bearings are formed with ball or roller bearings to reduce abrasive wear associated with friction. The inner and outer members of radial ball bearings each includes a groove, and each ball bearing is disposed within a groove of the inner member and a groove of the outer member. As ball bearings of the thrust bearing are worn and their diameters decrease, relative axial movement between the outer thrust members and the inner thrust members applies an uneven load on inner members and outer members of the radial bearing. Because of the radial bearing's arrangement with the ball bearings disposed within grooves in the outer members and the inner members, relative axial movement between the outer members and inner members is not allowed. Accordingly, this radial bearing arrangement fails as the thrust bearing is worn.